The present invention relates to a scanner motor and, more particularly to a scanner motor having a rotary polygon mirror rotating at high speed for use with an image forming apparatus, such as a laser beam printer or the like.
Conventional scanner motors of this kind are roughly classified as scanner motors using a ball bearing for a bearing portion and scanner motors using a fluid bearing for a bearing. There is a demand that these scanner motors can be operated at a higher speed and at a larger load as the image forming apparatus, such as the laser beam printer or the like, are operated at a high speed and a picture quality becomes high in resolution.
A bearing using a ball bearing is simple in arrangement and easy to assemble and control as compared with the fluid bearing and therefore its application range is widespread in the high speed and large load bearings. On the other hand, the fluid bearing needs parts of high accuracy and also needs an assembly circumstances in which an air is kept clean. Therefore, an oil SGB using an oil as a fluid receives a remarkable attention instead of an air bearing.
However, when the ball bearing or the oil SGB is effectively used in the bearing portion, it is frequently observed that a oil component is dispersed and volatilized from the bearing portion when the motor is rotated so that the oil component is overflowed to the outside of the motor to smudge a reflection surface of the rotary polygon mirror or optical devices disposed around the rotary polygon mirror. If the optical surface of the rotary polygon mirror is smudged by the oil component as described above, there is then the problem that an intensity of light is lowered to make an output image uneven in light and shape or that noise light is formed around a laser beam spot by scattered light, thereby a picture quality being deteriorated considerably.
Therefore, to solve this problem, there is proposed a technique which is disclosed in Japanese laid-open utility model publication No. 4-120919, for example. FIG. 4 shows a cross-sectional view of a scanner motor. Reference numeral 31 designates a bracket, 32 an electromagnet on the stator side, 35 a rotary shaft supported by upper and lower ball bearing supporting portions, 36 a permanent magnet on the rotor side, and 37 a rotary polygon mirror. According to this prior art, in order to prevent an oil component from being overflowed to the outside when the oil component is dispersed from or volatilized from the ball bearing supporting portion, the bracket 31 is provided with a concave portion 31a on which a rib portion 31b is formed. This rib portion 31b generates an air current flowing to the upper bearing supporting portion when the rotary shaft 35 is rotated in the direction shown by an arrow A. As a result, the oil component from the bearing supporting portion can be prevented from being dispersed or volatilized, thereby making it possible to prevent the rotary polygon mirror 37 from being smudged by the oil component. FIG. 5 shows a perspective view of the bracket 31 in which reference numeral 31c depicts a through-hole.
However, the above-mentioned prior art encountered with the problem that, when the rib portion 31b is exposed to wind generated by the rotation of the rotary polygon mirror 37, a large noise is generated by a resulting impulse.